PROJECT 3: Superinfection, or sequential acquisition of HIV variants, could spread viruses that are more drug-resistant or more virulent. The circumstances that govern susceptibility to superinfection are not known, and may differ in subtype B epidemics (like that in North America) compared with epidemics in Africa. A better understanding of superinfection risk, consequences, and biological determinants will help guide counseling of infected persons and may provide clues to protective immunity and HIV pathogenesis. Available case reports indicate that superinfection has occurred primarily in recent seroconverters. Using the same detection methods, superinfection was not observed in large cohorts of persons with established infections. We now propose to extend our research on superinfection to recently infected persons, who appear to have higher risk. We will specifically test the hypothesis that superinfection risk decreases over the course of HIV infection (aim1a) despite increases in exposure to HIV-1-infected partners (aim1b). Our findings suggest that serosorting, or preference for partners having the same serostatus, increases with increased duration of infection. The power to detect superinfection cases is enhanced by combining cohorts in San Francisco and Southeastern Brazil. We also will determine if superinfection, or dual infection, is associated with acceleration in disease progression, as indicated by plasma RNA level, CD4 T cell counts, T cell activation, and naive and memory subset analysis (aim 2). Inclusion of persons with multiple baseline infections helps assure power for analysis of laboratory markers of disease progression. We will also investigate biological mechanisms that may determine superinfection risk (aim 3). We will specifically test the hypotheses that superinfecting viruses may have greater replication capacity, greater fusion capacity, broader tropism, or increased capacity to escape from autologous serum neutralization or cell-mediated immune responses (in coordination with Project 4). To extend our recent findings, we will determine if serum neutralization responses become broader over time in a manner that correlates with superinfection risk. Viral phenotypes will be assessed using whole virus and viral test vectors. This project will fill important gaps in knowledge about superinfection and dual infection, which bear directly on the epidemic spread ofHIV, protective immunity, secondary prevention, and HIV pathogenesis.